Many reactions are hazardous and care needs to be taken to ensure no accidents. The more accurate and more timely monitoring of the reaction provided by this invention enables reactions to be performed within stricter limits. This enhances safety and can reduce the reaction inefficiencies that, hitherto, were an inherent shortcoming of the manufacturing process. Furthermore, the ratios of reactants can be optimised reducing the need for excess reactants to ensure completion of a reaction.
Reactions whether they be physical, chemical or both generate or absorb heat and there is therefore a heat change across the reaction. The theoretical heat generated or absorbed in a particular reaction is known from established information. The actual heat generated or absorbed during the course of a reaction could therefore, in theory, be a useful measure to determine reaction efficiency in the case of steady state reactions and reaction progress in the case of batch reactions.
By way of an illustration of the theory, a typical chemical synthesis step will be considered. Two reagents (A and B) react together to form a new compound (C) as follows:A+B Cwhere                A=kmol of A        B=kmol of B        C=kmol of C        
The heat generated by this reaction is established according to the formula:Q=Hr·C (kJ).where                Hr=heat of reaction per kmol of C produced (kJ/mol)        C=kmol of component C produced (kmol)        
The value of Hr may be determined from theoretical data or laboratory calorimeters.
Currently the heat data described may be used in a variety of ways.
For any reaction, the maximum theoretical heat liberation can be calculated as follows:Q′=Hr C′ (kJ)where                Q′=maximum theoretical heat generated (kJ)        Hr=heat of reaction per kmol of C produced (kJ/kmol)        C′=maximum theoretical yield of component C (kmol)        
The maximum theoretical yield C′ is based on the assumption that one or both of the feed components (A and B) are completely consumed.
If the heat of reaction is measured during a process, the quantity of component C synthesised at any time is as follows:C=Q/ Hr (kmol)where                C=quantity of C produced (kmol)        Q=heat measured during the reaction (kJ)        Hr=heat of reaction per kmol of C produced (kJ/kmol)        
Thus the total mass of C can be calculated by knowing the total heat absorbed or liberated and the heat of reaction (or crystallisation etc).
The expected theoretical yield of C is known from the quantity of reactants present and the stoichiometry of the process. Thus from the information above, the percentage conversion can be determined from the equation below.
                                 =                                    C              /                              C                ′                                      ×            100                                                        where          =                      percent            ⁢                                                  ⁢            conversion                                                        C          =                      quantity            ⁢                                                  ⁢            of            ⁢                                                  ⁢            C            ⁢                                                  ⁢            produced            ⁢                                                  ⁢                          (                              k                ⁢                                                                  ⁢                mol                            )                                                                                C            ′                    =                      maximum            ⁢                                                  ⁢            theoretical            ⁢                                                  ⁢            yield            ⁢                                                  ⁢            of            ⁢                                                  ⁢            component            ⁢                                                  ⁢            C            ⁢                                                  ⁢                          (                              k                ⁢                                                                  ⁢                mol                            )                                          
In batch reactions, percent conversion ( ) provides an effective means of identifying reaction end point and/or optimum reaction ratios. This can be used to reduce manufacturing time, improve plant utilisation, and reaction efficiency.
The present invention may also be used in laboratory activities such as in laboratory calorimetry. Use of the techniques of the present invention can reduce or eliminate the errors in conventional jacketed calorimetric measurement and simplifies temperature control during calorimetric measurement. In this way a quicker and more accurate method for the determination of theoretical Hr is provided. Unlike optical analytical devices, the calorimetric data is measured with inherently simple instruments which are not impaired by common process effects (fouling, composition change, temperature variation, mixed phases etc). Unlike optical analytical devices calibration of the calorimetric instruments is not product specific and instruments can be tested and calibrated on any fluid.
In continuous (plug) flow reactors, reaction efficiency ( ) provides a parameter for controlling feed rate to the reactor and controlling process conditions. In this way it is possible to run conventional batch processes in small-scale plug flow reactors. This benefits all aspects of the manufacturing process including lower capital cost for equipment, increased plant versatility, improved product yield, safer process conditions (through smaller inventories), greater product throughput and reduced product development time.
The ability to monitor reaction progress has an additional safety benefit for both small and large reactors. A system with online calorimetric data can instantly identify when unreacted compound is accumulating in the reactor. This reduces the risk of runaways due to accumulation of unreacted chemicals.
The design of reactors in common industrial use is however inherently unsuitable for measuring calorimetric data and thus the techniques described remain theoretical.
Chemical reactors in common use in, for example, the pharmaceutical and fine chemical industries fall into four main categories. Standard batch reactors in which reagents are mixed in a stirred vessel in which heat is added or removed by means of heat transfer fluid recirculating though an external jacket. These are the most commonly used reactors for small-scale organic and inorganic synthesis reactions. Batch reactors with internal coils, which are a variation on the standard batch reactor and have additional heat transfer surfaces within the body of the liquid. These reactors are used for general-purpose batch reactions where higher heat loads are encountered. Loop reactors in which reactants are pumped through an external heat exchanger and returned to the vessel. These are commonly used for gas/liquid reactions in which case the liquid is returned to the reactor via a spray nozzle to create a high gas/liquid interfacial area. Continuous reactors in which reactants are pumped through a heat exchanger under steady state conditions. These are generally used for larger scale manufacturing processes with long product runs.
The heat transfer characteristics of the four types of reactors described above have three common features:                i. The heat transfer fluid is circulated through the heat exchangers at high velocity to maintain favourable heat transfer coefficients. In the case of jacketed reactors, this is achieved by injecting the heat transfer fluid into the jacket at high velocities using nozzles or diverting flow around the jacket with baffles. In some instances, coils for the flow of heat transfer fluid are welded to the outside wall of the reactor vessel.        ii. High mass flow rates of heat transfer fluid are employed to maintain a good average temperature difference between the heat transfer fluid and the process fluid.        iii. The heat transfer area is fixed and temperature control of the process fluid is achieved by varying the temperature of the heat transfer fluid. In some cases limited scope exists for increasing or decreasing the heat transfer area.        
The features described above represent good design practice for achieving a flexible and optimised heat transfer capability within the reactor. However, these features do not lend themselves to measuring the quantity of heat generated or liberated. This deficiency is illustrated by reference to the chemical reaction between reagents A and B as discussed above. (It should be noted that the example is not limited to chemical reactions and is equally applicable to other chemical and physical processes).
When the two reagents (A and B) react together to form C, heat is liberated. The heat liberated per second can be expressed as follows:q=Hr·c (kW)where                q=heat liberated per second (kW)        Hr=heat of reaction per kmol of C produced (kJ/kmol)        c=kmols of component C produced per sec (kmol/s)        
If the process temperature remains constant the heat liberated (q) will be observed as a temperature rise in the heat transfer fluid according to the formula.q=m·Cp(tsi−tso)where                q=heat absorbed by the heat transfer fluid which is the heat liberated by the reaction (kW)        m=mass flow rate of the heat transfer fluid (kg/s)        Cp=specific heat of heat transfer fluid (kJ·kg−1K−1)        tsi=temperature of heat transfer fluid in (° C.)        tso=temperature of heat transfer fluid out (° C.)        
However, in order to determine q, the flow rate and temperature change of the heat transfer fluid (tsi−tso) must be measured accurately. In the reactor examples described above, effective design favours high flow rates of heat transfer fluid. Often this leads to a temperature change of the heat transfer fluid (tsi−tso) of less than 1° C. An IEC Class A RTD is one of the more accurate temperature measurement devices available. These devices have a tolerance of ±0.25° C. (the error on the installed device may be higher).
Thus for a temperature change of 1° C., the accuracy of heat measurement can be expected to be ±25% or worse. This would rise to 250% where the heat transfer fluid temperature changed by 0.1° C. This factor alone makes it virtually impossible to measure the heat of reaction in conventional reactors. Furthermore, on a conventional reactor, heat leaking out of the system via the non-process side of the jacket can create serious error.
Furthermore, conventional chemical reactors often have sluggish control systems which permit temperatures of the bulk material to cycle by a few degrees. In energy terms a few degrees change in temperature can represent a significant proportion of the overall energy release.
Furthermore the control speed is faster in that the ability to main the conduits at constant temperature permits a much higher correcting temperature on a newly opened conduit. The control is therefore faster and more accurate.
Conventional reactors offer acceptable heat transfer characteristics when the flow of heat transfer fluid is held at a good velocity. Since the heat transfer surface is limited to 1 or 2 discrete elements, the range (of energy liberated or absorbed) over which a useful service temperature rise (tsi−tso) can be achieved is very limited. In a case where the energy release from the process is small, the temperature rise in the heat transfer fluid may be a fraction of a degree. In addition to this, the shaft energy of the heat transfer pump could be a high proportion of the total.
The limitations described above are common to all reactors (and evaporators, batch stills etc) used in the pharmaceutical, chemical and allied industries. Accordingly, when employing these reactors the heat generated or consumed by the reaction cannot be used to monitor the progress of a reaction within any degree of accuracy.
It has been proposed in U.S. Pat. No. 6,106,785 that the heat generated in a polymerisation reaction may be used to monitor the progress of the reaction. The system of U.S. Pat. No. 6,106,785 is however a coarse method for monitoring a reaction which involves employing an inferential sensor, whose concept is based on the observation that for polymerisation processes, the amount of heat released is proportional, albeit in a non-linear way, to the degree of the monomer conversion. According to U.S. Pat. No. 6,106,785 by careful calculation of the reactor's thermal balance on-line one can continuously infer the degree of conversion and use it for control. Once the actual degree of conversion can be determined and ultimately controlled, one can also control the cooling duty of the reactor and thus make it conform with the cooling capacity allotted to it by the plant scheduler. U.S. Pat. No. 6,106,785 is therefore concerned with optimising the use of heat transfer fluid and the addition of initiator/inhibitor within safe operating parameters.
In U.S. Pat. No. 6,106,785 the batch controller data is used directly to control the reactor mixture temperature by manipulating the incoming coolant flow and temperature. The data are fed into the inferential sensor, where they are used to infer the current value of the degree of monomer conversion.
In U.S. Pat. No. 6,106,785 the degree of conversion is not therefore measured directly, but it is inferred by dynamically evaluating the reactor heat balance. U.S. Pat. No. 6,106,785 therefore enables one to infer the degree of conversion from the dynamic evaluation of the reactor heat balance. The use of the degree of conversion replaces special sensors for feedback control with respect to the product quality (end-use) properties. The use of the degree of conversion also replaces physical time for the timing of process related operations like valve opening and closing, and enables control of the heat supply/removal, dosing of the reactants, and so forth. The use of the sensor is said to allow an increase in the accuracy of the prediction of the batch evolution and thus enables a more accurate prediction of the cooling need profile than that provided by the systems previously used.
In U.S. Pat. No. 6,106,785, the reaction mixture temperature and the integral heat rate are treated as two independent process variables. This approach is said to allow the user the freedom to specify batch recipes in a way that defines the evolutions of either variable during the batch run, and to execute them under tight, high performance control. Because the degree of monomer conversion is proportional to the integral heat rate for many important polymers including PVC, controlling the two variables is said to allow the user independent control over two basic determinants of products quality. According to U.S. Pat. No. 6,106,785 this control fully defines the heat release at every instant of the batch run, thus making it possible to better utilize the available cooling capacity through more reliable planning and scheduling. To control the temperature and integral heat rate independently, the proposed method manipulates the amount of heat added to or taken out of the reaction and the amounts of the initiator(s) and inhibitor added during the batch run.
Whilst these techniques bring benefits in optimising the use of the coolant they are not sufficiently accurate and discerning to enable sophisticated sensing and control of a reaction. The present invention provides the solution to this problem.
In our co-pending United Kingdom Patent Application 0110301.9, we describe and claim reactor systems which provide improved control over physical and/or chemical reactions. The present invention relates to United Kingdom Patent Application 0110301.9 in that it enables the improved control to be achieved over a wide range of operating conditions by the use of a variable heat transfer area between the process fluid and the heat transfer fluid. Our co-pending United Kingdom Patent Application 0110295.3 describes measurement systems which may be used with this invention.